The present invention relates generally to power control. More particular, the resent invention relates to providing a controlled AC voltage to randomly-fluctuating loads such as electric arc furnaces.
Electric arc furnaces (EAFs) are widely used in a variety of applications, including the melting of scrap metal. EAFs represent erratic nonlinear loads to their associated power networks. This is because the arc is random and fluctuating in nature due to the unevenness of the material surface and the mechanical vibrations of the arc electrode assembly. EAFs can adversely affect power quality through, for example, poor power factor, harmonics, and "flicker".
Flicker occurs in the power network as a result of a rapidly changing system voltage, mainly during the melting of scrap metal in the electric arc furnace. When scrap is provided to the furnace, arcs between the furnace electrodes and the scrap melt the metal. The molten metal drips to the bottom of the furnace, and the arcs shift to other pieces of scrap metal. The shifting of the arcs results in highly variable and reactive power consumption. Further, short circuits can result from pieces of scrap falling onto the electrodes, thus shortening the arcs and causing the furnace electrode control system to reignite the arc. It should be apparent from this description that an electric arc furnace consumes a widely and rapidly fluctuating amount of supply current and voltage, thus causing flicker and inefficiencies in the power network. As the power ratings of EAFs have increased, these problems have increased in intensity.
Accordingly, it is generally desirable to stabilize arc currents in a manner which minimizes disruption to the electric power supply grid. It is also generally desirable to provide steady-state regulation of the arc current, and to improve power efficiency.
One method of controlling the arc current is shown in FIG. 1, where power is supplied to an EAF 10 from a supply 12 through a main transformer 14 having no-load taps on the load side, and a tap-changer. EAF current can be controlled in this arrangement by temporarily suspending the operation of the furnace, and adjusting taps on the supply transformer 14 to alter the voltage applied to the electrodes of the arc furnace 10. For example, a melt cycle begins on one tap, and at the end of the melt cycle the process is interrupted while the tap is moved to a more optimal location to complete the melting process.
FIG. 1 also includes a Static VAR Compensator (SVC) 16, which can be used to reduce the side effects of the randomly fluctuating nature of the arc load. SVCs involve shunt compensation on the electric supply to the EAF. The SVC is separate from the EAF, and is usually in a physically remote location. Typically, SVCs are rated significantly higher than the EAF load- SVCs can be up to 5 times the rating of the load (e.g., 250 MVAr SVC may be required for a 50 MW electric arc furnace).
A direct current (DC) EAF is shown in FIG. 2. The DC arrangement includes power electronics, in the form of a controlled thryistor bridge 18 rated for the full power of the load 10. This arrangement supplies the load with a controlled direct current, which can be regulated while the load DC voltage varies based on the arc condition. This DC arrangement provides relatively limited control, and a SVC (not shown) may also be required. Further, additional converters may be necessary to reduce the fluctuations of the reactive current on the power supply. Practical installations typically use at least two converters, with separate transformer windings, to achieve 12-pulse operation.
Another solution is shown in FIG. 3, where a controlled series reactor 20 is provided between the electric supply and the furnace transformer. The series reactor 20 has a controlled thryistor portion on the high voltage side of the transformer. The series reactor 20 is physically relatively large, and has a rating substantially equal to the load. The thyristors regulate the effective reactance imposed in series with the load to partially mitigate the fluctuations of the load current. This arrangement has relatively limited compensation ability, due to the load fluctuation caused by changes in the real part of the impedance. The series reactor can affect only the imaginary portion of the impedance. Again, this arrangement may also require a SVC (not shown) to provide improved regulation.
U.S. Pat. No. 5,617,447 discloses a method of stabilizing a power supply network against reactive load fluctuations, and a reactive power compensation device for a DC arc furnace. Inductors are used to provide reactive compensation. The disclosed technique features a relatively complex arrangement of multiple control loops, and multiplication of power control element outputs.
U.S. Pat. No. 5,610,937 discloses a method for regulating a DC arc furnace. To reduce flicker, comparatively fast reactive power regulation is mathematically superimposed on the relatively slow current regulation of the DC arc furnace.
U.S. Pat. Nos. 5,463,653 and 5,677,925 disclose a power converter device for DC power supply to an electric arc furnace. The power converter device includes at least one transformer in which the secondary winding applies a three-phase current to a rectifier circuit. The rectifier circuit is a non-reversible "freewheel" circuit, and includes controlled semiconductors for each secondary winding.
While the techniques described above provide voltage regulation, none provide sufficiently simple, effective, and low cost solutions to the problem of voltage regulation in a power grid which supplies power to an electric arc furnace.
In view of the above discussion, it would be desirable to provide a relatively simple, effective, reliable, and low cost voltage control capability for an electric arc furnace or other similar load on a power system.